Electrophotographic photoconductors used in copiers and printers are usually organic photoconductors that include a photosensitive layer containing an organic photoconductive material as a principal component. Such organic photoconductors are classified into two types: those having a single-layered photosensitive layer containing a charge generation material and a charge transport material; and those having laminated photosensitive layers in which a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material are laminated. Among these, the organic photoconductors having the laminated photosensitive layers, and particularly the negative charge type laminated electrophotographic photoconductors having the surface of the photoconductor to be negatively charged have been widely put to practical use because of their good electrophotographic properties, durability and high freedom of design.
The negative charge type laminated electrophotographic photoconductor usually includes a conductive support, an intermediate layer, a charge generation layer, and a charge transport layer, which are laminated in this order. When the negative charge type laminated electrophotographic photoconductor is light-exposed, it generates charges in the charge generation layer. Among the charges, the negative charges (electrons) migrate through the intermediate layer toward the conductive support, and the holes migrate through the charge transport layer toward the surface of the photoconductor and negate the negative charges on the surface of the photoconductor to form an electrostatic latent image. For this reason, the intermediate layer needs to: 1) quickly allow the electrons generated in the charge generation layer to migrate to the conductive support side (i.e., electron transportability), and 2) suppress injection of holes from the conductive support to the photosensitive layer (i.e., blocking property).
The intermediate layer usually contains metal oxide particles and a binder resin in which the metal oxide particles are dispersed. In order to improve the blocking property of the intermediate layer, increase in dispersibility of the metal oxide particles by surface treatment of the metal oxide particles has been studied. A variety of methods for surface treatment have been proposed: for instance, it is proposed to subject metal oxide particles contained in the intermediate layer to a surface treatment with anhydrous silicon dioxide (for example, PTL 1).